The present invention relates generally to alternating current motor drives and, more particularly, to a variable speed induction motor drive of the type using semiconductor devices capable of being turned on and turned off in response to control signals. Such drives include Gate Turn Off thyristors (GTO) and power transistors. The GTO is, however, becoming increasingly popular, generally has a higher power rating and it is toward systems employing GTOs that the present invention is particularly applicable, although not so limited.
There are a number of instances where it is desired to have a variable speed motor drive using an alternating current induction motor. Typically, these drives using alternating current (AC) to direct current (DC) source side converter connected to a polyphase (e.g., three phase) source of power. The output of the source side converter, voltage and current, is supplied via a dc unit circuit to a load side converter, normally referred to as an inverter which converts the DC link current into polyphase currents of variable frequency for application to the indicator motor load. Often, a large induction is included in the DC link circuit and the overall configuration is what is generally referred to as a current source inverter.
In such systems, the basic operation is that the source side converter is controlled to regulate the motor current through the DC link circuit while the load side inverter is frequency and phase controlled to regulate speed and motoring and braking torque. Both the converter and inverter are responsive to a basic speed or frequency command signal which is applied to two separate regulating channels, one for each of the converters. Control of such a system, however, may be lost when the source side converter is for some reason unable to maintain control of the DC link current. This can happen in a number of ways. For example, if a line disturbance occurs on the polyphase AC line source, the value of the line voltage may dip. If the dip is large enough, the maximum DC voltage which can be produced at the output of source side converter may be less than the DC voltage at the input side of the load inverter. In such condition, in a motoring mode, current in the link circuit will be reduced to zero. Even though the source side converter senses this condition, it cannot drive the current into the load side inverter. In a generating mode of operation, while the opposite situation exists, control is also lost with current larger, rather than smaller than desired.
Another condition under which control may be lost occurs at high speed under no load condition of the motor. Here the motor voltage on the line between the motor and the load side inverter rises to an extent that it can exceeds the AC source voltage. In this situation, the source side converter is unable to generate the DC voltage which is greater than the load voltage when motoring is again required causing current in DC link to fall to zero with subsequent loss of control.
It is known to provide a cross tie arrangement between the control circuits of the two converters such that when a potential loss of control situation exists, the system is operative to alter the firing angle of the load side inverter to regulate the DC link current in the event the source side converter is unable to maintain the 5 required current regulation. Such a system, applied to load commutated inverter drive, may be found in U.S. Pat. No. 4,420,719, "Cross-Tied Current Regulator for Load Commutated Inverter Drives" by John D. D'Atre et al. issued Dec. 13, 1983, which patent is assigned to the assignee of the present invention.
The system in that patent works quite well when the load side inverter is a standard thyristor type bridge, typically operating a synchronous motor, since such a system normally operates with the firing angle of the load side inverter at or near the limit, except at light load conditions or when transitioning from motoring to generating.
This limit load firing angle is set at the best load power factor angle and will still provide enough volt seconds for commutation of the load inverter thyristors. This cross tie arrangement, therefore, moves the load firing angle to a more conservative load firing angle, that is, one with more volt seconds for commutation. Also, the gain characteristics of the cross tie are predictable and equivalent to what effect the same amplitude signal would have had on a source converter.
In a GTO induction motor drive, however, the load firing angle is being varied continuously and the inverter is not normally operated in a clamp limit. This is particularly true in the case of the system in which notching is utilized to control harmonics. Injection of a cross tie signal as was done in the foregoing patent would cause the speed regulator to try to eliminate the contribution from the spillover of the cross tie circuit. That is, that particular scheme would not be applicable to a system of this nature.